Ethylbenzene is a hydrocarbon compound with high commercial utilization and value. It is majorly used to produce styrene which is an intermediate for polystyrene production. Ethylbenzene is commercially produced from alkylation of benzene with ethylene. However, the cost and competitive demands of ethylene and benzene prompted new efforts to recover ethylbenzene from various C8 aromatic feed streams which already contain ethylbenzene. Such feed streams are generally produced as a byproduct stream from several petrochemical processes and they usually contain ethylbenzene and one or more hydrocarbon compound with boiling point close to boiling point of ethylbenzene, especially C8 aromatic isomers.
Methods for separating ethylbenzene from other close boiling C8 aromatic compounds, especially xylene isomers, are known.
For example, U.S. Pat. No. 3,917,734 discloses an adsorptive separation process for separating ethylbenzene from a feed mixture comprising ethylbenzene and at least one xylene isomer, which process comprises contacting the feed mixture with a crystalline aluminosilicate adsorbent to selectively adsorb xylene isomers with substantial exclusion of the ethylbenzene and thereafter recovering purified ethylbenzene as a product. The separation efficiency of this process is largely relying on adsorption capacity of the adsorbent and therefore a complexed operation of countercurrent moving-bed system is preferred. Also, impurities content in the feed stream needs to be carefully controlled to prevent interference with the selective adsorption process.
Ethylbenzene can, of course, be separated from xylene isomers by fractionation but because there boiling points are very close to each other the fractionation can be achieved only with the more intricate and energy intensive system.
GB 1,198,592 discloses a process for separating C8 aromatic isomers using a single polyfunctional distillation column. The distillation is carried out in a multiplate column having at least 250 and preferably 365 trays and a reflux ratio from 100 to 250:1 in order to achieve commercially acceptable purity of ethylbenzene.
Extractive distillation is the method of separating close boiling compounds from each other by carrying out the distillation in the presence of an added solvent or so called an extractive agent. The presence of the said extractive agent in the distillation system alters the relative volatility and therefore makes possible a greater degree of separation of the close boiling compounds.
U.S. Pat. No. 3,105,017 discloses a process for separating ethylbenzene from xylene isomers by extractive distillation in the presence of a compound containing a single benzene ring substituted on the ring in at least two positions with a chloro group under condition to separate a fraction enriched in ethylbenzene.